uncultured prokaryote Raw sequence reads

Pelagic microbial communities of streams represent unique populations that are distinct from the microbial communities of the stream benthos and surrounding soil. In this study, we utilized high-throughput 16S rRNA gene sequencing to characterize microbial community composition within a temperate stream network. We found that the microbial communities of headwater streams are compositionally diverse, with low representation of known freshwater microbial taxa. In three out of five seasonal samplings, a successional pattern was identified in which phylotype richness and compositional heterogeneity (cross-stream and over time) decreased while the proportion of known freshwater taxa increased with increasing stream size. However, this trend was not observed in two seasons. Instead, streams exhibited uniformly high microbial diversity across the watershed, and the fraction of freshwater taxa showed no relationship with stream size or distance traveled. Our data suggests that longitudinal trends in microbial diversity in temperate watersheds are generated by microbial dispersal from surrounding environments into stream headwaters, followed by in-stream selection for specific freshwater-adapted microbes. However, this microbial succession appears to be a highly dynamic process that can be disrupted at landscape scales, potentially in response to variation in temperature and precipitation.